This invention relates to an optical directional coupling arrangement and, more particularly, an optical directional coupling technique which is adapted to translate a linearly polarized mode into a substantially randomly polarized mode, through a polarization maintaining single mode fiber and then permitting the resultant mode to be incident to an optical fiber to be measured.
As a light measuring apparatus, for example, a light pulse testing apparatus is known which is adapted to measure a fault on an optical cable by measuring back-scattered light which is reflected back in the optical cable after its original incident light has fallen into the optical cable. Such an optical pulse testing apparatus uses an optical directional coupling apparatus as shown in FIG. 1. In FIG. 1, a light of the linearly polarized mode which has been incident to a first port 1 of an optical directional coupling device 6 from, for example, a laser diode (not shown), is sent through a polarizing element (for example, a polarizing prism), a second port 3 of the optical directional coupling device 6 and a polarization maintaining single mode fiber 4 to an optical fiber 5. The optical directional coupling device 6 is comprised of the first port 1, the polarizing prism 2, the second port 3 and a third port 12. The optical directional coupling device 6 and the polarization maintaining single mode fiber 4 constitute an optical directional coupling apparatus 9. A light fiber 7 indicates a light fiber introduces light which is reflected back as back-scattered light into a light receiver (not shown) and an optical connector 11 is provided.
The light of the linearly polarized mode which has been incident to the first port 1 is sent to the optical fiber 5 through the optical directional coupling apparatus 9. When the optical fiber 5 is of a multimode type, the light of the linearly polarized mode which is sent to the optical fiber 5 has its linear polarization characteristic readily cancelled due to the characteristic of the optical fiber and, at the same time, the light which is reflected back as back-scattered light has its linear polarization characteristic also cancelled, thus making substantially constant the polarization direction components of the back-scattered light which are branched through the polarizing prism 2 of the optical directional coupling device 6 and which are conducted to the third port 12. When, on the other hand, the optical fiber 5 is of a single-mode type, the above-mentioned light has its linear polarization characteristic substantially unaffected, unlike the case of the multi-mode optical fiber, due to the characteristic of the optical fiber and the back-scattered light which is reflected back in the optical fiber 5 with its linear polarization characteristic also substantially unaffected. In practice, however, the linearly polarized mode is translated into an elliptically polarized mode, depending upon the length of the optical fiber 5, a stress inflicted upon the optical fiber 5, and elliptic deformation, anistropy, heat, etc., and thus the vertically polarized component X of the elliptically polarized mode as shown in FIG. 2 is branched through the polarization prism 2 of the optical directional coupling device 6 and conducted to the third port 12. Whether the major axis of the ellipse pattern of the elliptically polarized mode is inclined in any particular direction anywhere along the length of the optical fiber, or whether the major axis of the ellipse pattern of the back-scattered light as mentioned above is inclined in any particular direction, is not determined and the back-scattered light which has been received at the light receiving unit has its light receiving level varied along the length of the optical fiber, causing it to waver in a zig-zag fashion and thus posing an undesirable problem from the standpoint of measurement.